Split stdout and stderr on interactive execution

When running a command interactively, we previously would get stdout
and stderr both served on stdout by Nomad. To circumvent this issue,
we now start a separate execution inside the allocation to split
both streams.

configuration.example.yaml

@@ -12,6 +12,8 @@ server:
   certfile: ./poseidon.crt
   # The path to the key file used for TLS
   keyfile: ./poseidon.key
+  # If true, an additional WebSocket connection will be opened to split stdout and stderr when executing interactively
+  interactiveStderr: true
 
 # Configuration of the used Nomad cluster
 nomad: